An organic electroluminescence element (hereinafter, also referred to as an organic EL element) includes a pair of electrodes and a thin film containing an organic electroluminescence material (hereinafter, also referred to as an “organic EL material”) located between the pair of electrodes. Such an organic EL element emits light as follows. A singlet exciton, generated by recombination of a hole and an electron caused by a host molecule in the light emitting layer, causes energy transfer to a fluorescent molecule, which is a light emitting dopant. As a result, the fluorescent molecule emits light. The intensity of light emission of the organic EL element is controllable by the level of voltage to be applied or by the amount of electric current flowing in the element. A display device including a display screen that includes pixels formed by use of this characteristic has been developed.
A display device using an organic EL element is capable of displaying an image by controlling light emission of individual pixels independently. Therefore, the display device does not require a backlight unit, which is required in a transmission-type liquid crystal display device, and thus is made thinner. In the meantime, for producing such an organic EL display device, a combination of a dopant material and a host material suitable for the dopant material needs to be selected to form a light emitting layer for each of colors of light to be emitted. Therefore, a light emitting layer needs to be formed for each color of light to be emitted. Namely, different dopant materials and different assist dopant materials need to be provided in pixels corresponding to different colors. In the case of, for example, forming pixels of three colors of RGB, three dopant materials and three host materials suitable for the three dopant materials need to be prepared to form the pixels of the three colors.
For example, Japanese Laid-Open Patent Publication No. 2009-111023 describes the following. A single mixed layer containing a host material and at least three light emitting materials providing different colors of light is continuously formed in a display region. Pixels corresponding to each of the colors are irradiated with electromagnetic waves, so that the light emitting capability of the light emitting material that does not contribute to the color for the corresponding pixels is lost.
Japanese Patent No. 4281308 describes that one, same host material having a peak emission wavelength of 415 nm or shorter is used for all the light emitting layers and that an iridium complex is used as a dopant.